If molecules in an etching liquid react with a material to be etched, there is no reason for them to react more with the bottom of the material to be etched than with the side of a hole created in that material. This results in what is called under-etching: an etch that takes place under the side of a mask. Under-etching is often a limit to a semiconductor technology and needs to be controlled.
Under-etching experiments typically require the measurement of the horizontally etched distance underneath a film, e.g. a mask that partially covers the etched material. This cover material, when under-etched, is generally referred to as the structural layer in surface micro-machining, while the material etched away underneath the structural layer is generally referred to as the sacrificial layer. Different techniques can be adopted in order to measure the amount of under-etching so that the under-etch rate of the sacrificial material can be calculated.
Cross-section scanning electron microscopy (XSEM) is a reliable and accurate technique which allows a direct measurement of the under-etched distance 10 or the amount of top layer undercut (FIG. 1). However, sample preparation may be a tedious process because sample cleavage is needed at a specific location, without destroying the under-etched structure.
Measurement of the undercut through top view imaging becomes possible when the top layer is transparent or when it is removed. When the top layer is transparent, optical microscopy can directly be used. In the literature, several test structures such as etch channels, etch holes, etch plates and a square matrix were proposed for this purpose, for example in M. Van Oort and M. Verwillegen, “Strain diagnostics- and etch technology mask set for surface micro-machined structures”, Master's thesis, Hogeschool van Utrecht, UMECC, Utrecht Micro engineering Competence Centre, 2000; in D. J. Monk, D. S. Soane, and R. T. Howe, “Hydrofluoric acid etching of silicon dioxide sacrificial layers”, J. Electromech. Soc., 141(1):264-269, 1994; and in R. Hellin Rico, B. Du Bois, J-P. Celis, and A. Witvrouw, “Characterization of the etching of Ge sacrificial layers for surface micromachining of MEMS”, In Proceedings 15th Micromechanics Europe Workshop, pages 115-118, 2004.
If the top layer is not transparent, both optical microscopy (FIG. 2a) and top view SEM (FIG. 2b) can be used, but extra sample preparation is necessary as the top layer needs to be removed. Removal of the top layer can be done by using a selective dry or wet etch process that removes only the top layer without affecting the material under investigation, by scratching off the top layer away with a sharp tip, or by tearing the top layer off with adhesive tape. All these methods are thus destructive. FIG. 2a shows an optical microscopy image of a narrow line of sacrificial layer material 20 which is made visible after tearing off the structural layer (top layer) 21 with adhesive tape. FIG. 2B shows a top view SEM image which offers higher resolution than the optical microscopy image (FIG. 2a). One can distinguish the narrow line of sacrificial layer 20 material and some debris of the structural layer 21.
As mentioned in Tom Cheyney, “More important, more complex: Mems metrology”, Solid State, March:56-58, 2008, infrared inspection could be used to monitor undercut after sacrificial etching underneath Si and poly-Si in a non-destructive way. Although this technique can be used as inspection tool in, for example, the inspection at various stages of packaging as described in M. K. Shell and Golwalkar S., “Applications of infrared microscopy for bond pad damage detection”, IEEE/RPS, pages 152-159, 1991 and in A. Trig, “Applications of infrared microscopy to IC and MEMS packaging”, IEEE Transaction on electronics package manufacturing, 26(3):232-238, 2003, its application as a metrological instrument for undercut determination remains vague.
Another alternative to monitor the under-etched distance is the use of acoustic microscopy. This technique makes use of the large difference in the speed of sound between the liquid etchant and the solid material that is being etched, as described in D. J. Monk, D. S. Soane, and R. T. Howe, “Hydrofluoric acid etching of silicon dioxide sacrificial layers”, J. Electromech. Soc., 141(1):264-269, 1994. This technique allows in-situ monitoring of the etch front movement.
All the described approaches for under-etch determination are either destructive and/or time consuming Furthermore, these approaches are not able to generate large amounts of data in a short time.